The present invention relates to a technique for molding and obtaining a biodegradable article such as seed-rasing container, an non-electrostatic article such as packaging container for delivering a compact disc, a tray for an IC and the like, which comprises papers such as waste papers, wood-pulp papers and the like.
(1) Molding Compound
Hitherto, when an article including a waste paper was molded by using a conventional molding composition, there was a problem during the kneading step that the molding compound is easy to stick onto a wall of a kneader, a die and the like, resulting in poor workability.
Also in a step of drying and solidifying the molding compound charged in a cavity of a mold, the surface of the composition in contact with the wall of the cavity is dried and solidified first to form a surface layer. Further, since the surface layer is very thin immediately after completion of the charging, there is a possibility that the surface layer adheres to the wall of the cavity when a vapor pressure or a charging pressure is applied to the surface layer. In such a case, water added into the molding compound is vaporized by the mold which is heated to a high temperature. And it is difficult to discharge the water vapor by a degassing means through the interface between the wall of the cavity and the surface layer.
Further, if the surface layer adheres onto the wall of the cavity, there is a problem of crack development in the molded article due to frictional resistance with the wall of the cavity when a solidified molded article is released.
Namely, it is eagerly desired that a molding compound is not liable to stick onto a wall of a kneader during the kneading step and is facilitating water discharge during the molding step. Further, it is required that the molding compound provides a molded article which is easily released from the mold and does not generate a crack.
(2) Molding Method
In the conventional method of molding an article including a cellulose fiber, a molding compound obtained by adding and kneading a pulverized waste paper, starch and water is charged into a cavity of a heated mold and, then, dried and solidified by discharging a water vapor from the molding composition.
As means for discharging the water vapor, there is suggested a flash-type mold which is provided with degassing means with micro pores in the wall of the cavity (for example, refer to JP-A-9-76213). When the molding is conducted with such a flash-type mold, it is suggested that the charging pressure is increased to obtain a molded article having superior transferring property, dimensional accuracy, low air permeability and high density (for example, refer to JP-A-9-109113). In such case, a positive mold is employed to prevent the molding compound from flowing out from the cavity even if the mold is opened slightly. And, after charging the molding compound while opening the positive mold slightly, a part of the water vapor is discharged by means of degassing means. Then, the remaining water vapor is discharged after the mold is closed.
Further, a method for discharging the water vapor without providing degassing means when an article including a cellulose fiber is molded (for example, refer to JP-A-10-29250). In this method, it is suggested to open the mold to form a gap of about 1 mm and to discharge the water vapor through the gap in the drying step. It is noted that the molding compound comprising 3 to 100 parts by weight of gluten as a binder and 20 to 150 parts by weight of water per 100 parts by weight of raw paper material is used.
In the method of molding by employing the mold provided with degassing means having micropores, if the cavity has a portion to mold a thin molded article or a narrow groove to mold a rib, it is necessary to increase flowability of the molding compound in the cavity by applying a high charging pressure in order to charge the molding compound into a cavity uniformly.
On the other hand, the molding compound charged in the cavity is heated and dried from the surface portion which is in contact with the wall of the cavity having a high temperature. Therefore, a solidified surface layer (hereinafter, referred to simply as "surface layer") is formed on the surface portion of the molding compound. But, since heat transfer is delayed in the center portion of the molding compound, the molding compound is in the flowable state with a relatively low viscosity in the center portion. Since the surface layer formed of the molding compound is soft in the form of thin film, the surface layer is easily fractured due to an increased charging pressure and the flowable molding compound in the center portion flows into micropores of the degassing means to close the micro pore. Namely, there is a problem that it becomes difficult to dry and solidify the molding compound rapidly because the discharge of the water vapor is inhibited.
Further, in order to shorten the period of time for drying the molding compound, it is necessary to set the temperature of the mold to not lower than 120.degree. C., preferably not lower than 130.degree. C. to facilitate the generation of the water vapor. However, in the method wherein the water vapor is discharged by opening the mold to form the gap of about 1 mm without providing the degassing means having micropores, the molding compound flows into the large gap of the mold and a flash is easily formed due to the water vapor pressure when the temperature of the mold is set to not lower than 120.degree. C. Also, there is a tendency that a void is easily generated in the cavity because of the lack of the molding compound. The thickness of the molded article obtained by employing such a kind of a cellulose fiber is usually so thin that it is really disadvantageous if a flash or a void of about 1 mm is formed in such thin molded article.
On the other hand, when the temperature of the mold is not higher than 120.degree. C., it is necessary to increase an adding amount of water to make gluten perform the binding property and to give flowability to the molding compound. However, even if the amount of water per the solid content of cellulose fiber and gluten is decreased, the molding compound has flowability and the flash is liable to be developed. Therefore, if the amount of water is large, there are problems that the flash is further easily formed and that the molding cycle may not be shortened because the temperature of the mold is low and the drying period of time becomes longer.
Namely, it is eagerly expected that the molding method can be conducted without providing the degassing means with micropores and without generating the flash wherein the water vapor can be discharged smoothly and reliably in a short period of time and the charging pressure can be increased.
(3) Mold
Hitherto, when a molded article is molded, the shape of a cavity of a mold is fabricated on the basis of the outer shape of the article to be obtained and the molding compound is charged into the cavity, followed by solidifying the molding compound.
Apart from the case wherein the surface is treated by forming an irregularity, the wall of the cavity in a general mold is made smooth by grinding or is made mirrorlike by mirror polishing in order for the charged molding composition is liable to flow.
When the molding compound is charged into the conventional cavity with a smoothed surface or mirrorlike surface, the molding compound is cooled from the interface between the wall of the cavity and the molding compound in the case that the molding compound comprises a thermoplastic resin. Therefore, as described above, there is formed a solidified surface layer at the surface portion of the molding compound and, the inner portion of the molding composition wherein heat transfer from the wall of the cavity is delayed is in the flowable state with a relatively low viscosity. Since the surface layer formed on the surface portion of the molding compound is adhesive to the cavity wall, the molding compound is charged as the fountain-flow as shown in FIG. 1 to provide a homogeneously molded article.
Namely, the molding compound C is charged into the cavity B of the mold A, there is formed the surface layer E at the surface of the molding compound in contact with the wall D of the cavity B. Since the surface layer 5 is adhesive to the wall D of the cavity B, the surface layer E does not slip on the surface of the wall D.
Then, when the molding compound is charged into the cavity B, the molding compound is solidified at the surface portion which is adjacent to the wall D first. However, since the inner porion of the molding compound which is apart from the wall D via the surface layer E has flowability and may flow, the molding compound at the top of the flowing molding compound flows toward the wall D of the cavity B to form the surface layer E successively. The formerly charged molding compound forms the portion of the molded article corresponding to the portion around the gate and the lately charged molding compound forms the inner portion and the front portion of the molded article.
In the case that the molding compound comprises a thermosetting resin, the surface layer is not formed at the surface portion of the molding compound and there is flowability from the inner portion to the front portion of the molding compound. Therefore, the molding compound is charged as the plug-flow, which means that the inner portion and the front portion of the molding compound flow at almost the same velocity theoretically to give a homogeneously molded article.
On the other hand, in the case that a molding compound obtained by kneading a cellulose fiber added with at least a water-soluble binder and a large amount of water is molded by a heated mold, the molding compound is dried and solidified at the same time when the molding compound is in contact with the wall of mold. And, the surface layer is formed at the surface portion of the molding compound in almost the same manner as in the case of the thermoplastic resin.
However, in this case, there is the water vapor at the interface between the formed surface layer and the wall of the cavity. The water vapor is generated from the large amount of water contained in the molding compound. Therefore, adhesion between the surface layer and the wall of the cavity is remarkably lowered. As the result, the surface layer is pulled or forced by the flowing molding compound due to the charging pressure and slips easily. According to the phenomenon, there is a problem as described in the followings.
Namely, when the molding compound including a cellulose fiber, at least water-soluble binder and water are charged into the cavity having a ground or polished wall, the molding compound in the cavity flows in the manner of slip-flow as shown in FIG. 2, which means that mainly the surface layer flows while slipping. This is because the surface layer formed on the surface portion of the molding compound is less adhesive to the wall of the cavity.
The molding compound C charged in the cavity B of the mold A forms the surface layer E at the interface between the wall D of the cavity B and the molding compound. However, the surface layer E is less adhesive to the wall D of the cavity B, the surface layer E is pulled by the molding compound which flows with the charging pressure for example and, therefore, proceeds in the cavity while slipping on the surface of the wall D. As the result, the molding compound formerly charged in the cavity B forms the front portion of the molded article to be obtained. On the contrary, the molding compound lately charged forms the portion around the gate.
The phenomenon of the slip-flow may not be observed around all of the wall D of the cavity B. In case where the degree of the mirrorlike state on the wall of cavity varies depend on the portion of the wall, there are two portions existed, i.e. the portion where the surface layer is facilitating to slip and the portion where the surface layer is not liable to slip. Further, since the wall D of the practically manufactured cavity is not necessarily smooth, there are also two portions existed in this case. On the portion where the surface layer is facilitating to slip, the molding compound is charged with little resistance, the distance that the molding compound flows becomes long. On the portion where the surface layer is not liable to slip, on the contrary, there is a problem that the distance that the molding compound flows becomes short, resulting in a uneven charging since the flow rate of the molding compound becomes low.
Also, there is another problem that, near the completion of the charging, the portion where the flow of the molding compound is delayed begins to flow and forms a weld line with the formerly charged molding compound. There is further a problem that the molding compound at the weld line is not liable to be combined confirmly and the strength of the weld line is lowered to develop a crack when the mold is heated to a high temperature. This is because the front portion of the flowing molding compound is dried to some degree and fibers in the compound are difficult to entangle with each other in the weld line.
Further, if the flowing distance is long in case of a relatively large molded article, the formerly charged molding compound forms the front portion of the flowing molding compound by the slip-flow. Accordingly, when the front portion flows a long distance as it is, the front portion is dried to lose the flowability if the temperature of the mold is high. As the result, there are problems not only that the flow is stopped but also that the mechanical strength of the weld line is remarkably decreased and the obtained molded article becomes fragile because the dried molding compound is difficult to adhere and combine to each other at the weld line. Further, there is a problem that the obtained molded article has a void or a lacked portion therein when the flow is stopped during the step for forming the weld.
Namely, it is eagerly desired that the mold does not cause the slip-flow when the molding compound is charged.